Camshaft Timing Concept Question - 3.2L

[Gordo2]

I having a hard time picturing how cam timing adjustments work it as relates to the rotation of the inner and outer camshaft sprockets.

In part, this question relates to my tight fitting sprockets (both inner and outer sprockets required heating to install on the end of the cams). Thread related to that issue for additional reference:
Difficulty Installing Camshaft Woodruff Key and Sprockets - 3.2L

The Basic Concept?

I haven't timed 911 cams before, but if I understand correctly:

- The inner sprocket is fixed to the camshaft via woodruff key
- The outer sprocket is fixed to the crankshaft via timing chain
- The bolt and concentric washer at the end of the camshaft, lock the outer and inner sprockets together - the pin that goes between the two sprockets holds their relational position / timing setting while you tighten/torque the bolt at the end of the camshaft to fix them in place.

If the two sprockets are not locked together (bolt loose, pin removed), you should be able to rotate the crankshaft a small amount (outer sprocket rotation), while the camshaft (inner sprocket) remains relatively stationary / does not rotate.

This allows you to adjust the relationship between the crankshaft position and the camshaft position - set the timing of the camshaft to open/close certain valves at a relatively precise point in the crankshaft's rotation.

Am I good so far?

The Questions

#1 Shouldn't the outer sprocket be able to rotate independent of the inner sprocket (camshaft) in order to adjust the timing?

#2 And if so - wouldn't the tight fit / binding of my outer sprocket cause the camshaft to rotate as I rotate the crankshaft (even though the sprockets are unlocked)?

#3 Same question considering a highly tensioned chain: wouldn't the chain tension tend to bind the outer sprocket to the camshaft making it rotate with the crankshaft?

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Steven Stomski's cam timing video Timing Camshafts on a Porsche 911 Engine using STOMSKI RACING's SR098 - YouTube (at ~6min 30 sec point) prompted question #2. He says the chain tension (from his mechanical tensioner) holds the camshaft in place.

This seems counterintuitive to me - unless the tension of the chain causes the camshaft to bind in the camshaft tower to hold it in a static position.

This one's blowing my mind.

Appreciate your input.

Thanks, Gordo

[TT Oversteer]

Yes, the outer sprocket must be able to rotate in place by a small amount to adjust the cam timing accurately. You just need enough tension on the chain to preload any slack so that the timing value you set doesn't change under normal chain tensioner loads. This amount of chain tension will not prevent the outer sprocket from turning while setting timing. You don't want to put too much tension on the chain while setting timing because then the timing will change when you relieve the tension. Experiment with different amounts of chain preload and see how it changes the timing value. I think the best thing to do is to approximate the amount of tension that the engine has under normal running conditions: not loose but not super tight either.

I'd be a little concerned if your outer gears need to be heated and driven onto the camshaft. Maybe dress the cam with some emery cloth and oil so the gear will slide on and rotate by hand? Does Camgrinder apply a surface finish or coating to the cams after regrinding? Could that have slightly increased the diameter of the shaft that the gear slides onto?

This needs to be addressed as you will not be able to accurately set cam timing if force is required to move the outer gear on the camshaft. The locating pin can be inserted into an aligned hole in the sprocket and the gear will still have some degree of movement. It's the clamping force of the bolt that actually holds the alignment, not the pin.

[TT Oversteer]

Just want to add: If you're having trouble keeping the camshaft from moving while setting the timing due to friction between the sprocket and cam, you could try installing several or all of the rocker arms. This will add quite a bit of resistance to turning the cam. If you do this just be careful to get the relationship of the cam to the crank close before rotating the engine as you could bend or damage valves.

[Flat6pac]

As long as your loading the rockers, back the crank about 30 degrees BTDC and that will move all the pistons out of the way and youll be able to turn the cam shaft to load the rockers with out worry of touching the valves.
Bruce

[Gordo2]

Thanks for the suggestion folks,

I continued to pour through the seemingly endless number of cam timing threads and found one that had what appeared to be a few additional suggestions.

Cam Timing Question

Agree TT - may do a light polish if it's really stuck.

Gordo

[Tom '74 911]

I've found this video to be super helpful... Even if you're not using the Stromski tools, watching his other videos is helpful too as he explains a lot of the pre-cam timing set up etc...

Timing Camshafts on a Porsche 911 Engine using STOMSKI RACING's SR098 - YouTube

[KTL]

It's not uncommon for the large sprocket to stick on the cam snout. Not to mention it is difficult to hold the cam snout on the later style bolt-end cams. The earlier nut-end cams have the flattened snout that you can hold with the special socket or a suitable crescent wrench. But on the bolt-end you typically need something to turn the cam while you hold the sprocket steady.

I found the bolt-end cams to be pesky when the sprocket would not slip on the cam snout with the pin removed. I used to attempt setting the cam to a position that anticipates that inability to rotate it with the pin out and nothing to grab onto. It was a frustrating vicious cycle of back and forth fiddling that didn't allow me to precisely set the cam where I wanted it.

I think it was Bruce or Mike Bruns who suggested to me a few years back to insert the bolt with the big washer removed and snug it in place by counterholding the sprocket with the special sprocket holding tool. This bottoms the bolt in the end of the cam.

No need to gorilla tighten it. Just tight enough to allow you to use the hex head of the bolt & rotate the cam w/out the bolt loosening. Kind of like double-nutting. Then you can use the bolt head to rotate the cam forward or back to set your desired lift spec. Put the pin back in, put the special sprocket holding tool back in the sprocket to allow you to loosen the bolt. Remove bolt, put big fat washer on, put bolt back in. Torque it down, rotate engine back to TDC overlap and check the lift on dial indicator.

[lindy 911]

Originally Posted by Gordo2

Thanks for the suggestion folks,

I continued to pour through the seemingly endless number of cam timing threads and found one that had what appeared to be a few additional suggestions.

Cam Timing Question

Agree TT - may do a light polish if it's really stuck.

Gordo

Now you're cook'n.

[yelcab1]

All the 3.2L engines I have seen, the outer sprocket falls out when the bolt is removed. If you need to apply heat to remove it, you need a new sprocket or a new camshaft. Does it happen on both sides? or just one side?

I had this problem on the right side cam. When I remove the pin, and turn the crank to turn the outer sprocket relative to the other smaller sprocket, they stick and then to move together. I just move the crank shaft to where it needs to be, stick a small flat head screw driver into one the holes on the sprocket, gently pry the inner sprocket so that the dial indicator reads the right value, insert the pin, I am then good to go.

[Gordo2]

Originally Posted by KTL

... insert the bolt with the big washer removed and snug it in place by counterholding the sprocket with the special sprocket holding tool. This bottoms the bolt in the end of the cam.

This sound like the best option - thanks Kevin.

Tom, agree - great link, but I think I've seen it before ... Truth be told, I too am guilty of glossing over wordy posts (hint - the link to the Stomski video is toward the bottom of my original post ).

Thanks again folks,

Gordo